A secondary battery refers to a device which converts external electric energy into the form of chemical energy and stores it therein, and then generates electricity as necessary. Such a secondary battery is also called the name of ‘rechargeable battery’ which means a battery capable of being charged many times. Typical secondary batteries include a lead storage battery, nickel cadmium (Ni—Cd) battery, nickel metal hydride (NiMH) battery, lithium ion (Li-ion) battery and a lithium ion (Li-ion) polymer battery. A secondary battery provides both an economical advantage and an environmental advantage as compared to a disposable primary battery.
Currently, a secondary battery is used for some applications requiring low electric power. For example, such applications include a device that helps starting of a car, portable system, instrument and a no-brake power supply system. Recently, development of wireless communication technology leads popularization of a portable system. In addition, there is a tendency to convert many conventional systems into wireless systems. Under these circumstances, there is an exploding demand for secondary batteries. Further, hybrid cars and electric cars have been commercialized with a view to preventing environmental pollution. Such next-generation vehicles adopt secondary batteries to reduce the cost and weight and to increase the service life.
In general, secondary batteries are generally provided as cylindrical, prismatic or pouch type batteries. This is because secondary batteries are manufactured by installing an electrode assembly including a positive electrode, a negative electrode and a separator into a cylindrical or prismatic metal can or a pouch type case made of an aluminum laminate sheet, and then injecting an electrolyte to the electrode assembly. Therefore, a predetermined space for installing a secondary battery is required essentially. Thus, such cylindrical, prismatic or pouch-like shapes of secondary batteries undesirably function as limitations in developing various types of portable systems. As a result, there is a need for a novel type of secondary battery which allows easy deformation.
To meet such a need, a cable type secondary battery, which has a significantly larger ratio of length to sectional diameter, has been suggested. The cable type secondary battery requires a certain degree of bending characteristics to accomplish its purpose of use, and generally requires a level of 5R of bend radius.
Particularly, when applying a cable type secondary battery to electric wires for Bluetooth earphones requiring high bending characteristics, it is required to integrate an inner electrode, separator and an outer electrode in order to prevent an increase in contact resistance in the cable type secondary battery.